The first step in carrying out injection molding by the use of a molding machine in which an injecting cylinder is provided interiorly of a cylinder involves measurement of molding material required for one shot.
Ordinarily, a screw is rotated by applying back pressure thereto, and the quantity of material is measured. When the screw is moved backward to a predetermined position, rotation of the screw stops. If a distance of the screw from a position at which the screw starts its rotation to a position at which the screw stops is constant, it is considered that a quantity of the measured material is also constant. In addition, since the screw is moved backward by internal pressure of the cylinder produced by the rotation of the screw, the position of the backward movement of the screw is electrically controlled, and the stopped position of the screw is established as a measurement-completion position, which position is used as a reference to control pressure, speed, retaining pressure, position and time etc. in the injection steps.
However, even if the screw is immediately stopped by means of a device actuated by a signal from a measurement completion detector, internal pressure still remains in the cylinder, and therefore the screw is affected by the internal pressure. In the measurement completion position, back pressure applied when measurement is made is reduced down to such an extent that material is not discharged from the tip of a nozzle. Such a state is maintained until the subsequent injection step starts. Because of this, the screw continues to be moved backward by the internal pressure even after the screw has been stopped from its rotation. With this backward movement, the molten material around the screw enters into the front portion of the screw from a clearance between a check valve in an open state at the end of the screw and a valve seat, and measurement is naturally carried out even after completion of measurement and the quantity of material is increased. In addition, the internal pressure varies with the plasticized state and density of material around the screw, and variation in measurement after completion of measurement, that is, the amount of backward movement of the screw is not repeated.
Actually, when a number of products are molded at a time, unacceptably molded products result. It has been found by tests conducted to find the accuracy of material measured under the fixed stroke in an accurately controlled condition, that an error in weight of .+-.1.5 occurs on average.
The aforesaid error in weight is a weight % in which material of approximately 80% of measurement required for one shot of a mold is measured, and the total quantity thereof, that is, material is injection filled into the mold to the termination point of the injection stroke, and molded products which are insufficient in filling are repeatedly molded dozens of times under the same condition.
It has been found by the present inventor that when normal material and regenerated material (that is material which has been used previously for molding and which has irregular particles therein) are respectively injected under the same condition the regenerated material experiences a great weight error.
The present inventor has found from the foregoing that the variation in injection quantity not only results from the responsiveness during the closure of the check valve but is greatly affected by the density and plasticized state of material from a material inlet at the rear of the cylinder to the end of the screw at the front of the cylinder, whereby the quantity of molten material to be measured at the front of the screw is assumed to be varied; and the quantity of material injected on every shot cannot be quantitized on account of the above mentioned problems.